Question

Answer

Restrict your word count to only a few hundred words. I recommend 600-800

Taking those together, I suggest the following font sizes are a good starting point:

Titles 72+
Sub-titles 54+
Text 30+
Sub-text 20+

Obviously the actual font used can effect this. I tend to keep to a standard sans-serif font such as Arial or Helvetica (or my favourite Trebuchet). The values are, as I say, a starting point and will need to be changed (usually increased) to suit the design of the poster.